Magnetic memory element utilizing spin transfer switching

ABSTRACT

A magnetic memory element includes a pinned layer, a tunneling barrier layer, a free layer and a stabilizing layer. The tunneling barrier layer is disposed on the pinned layer. The free layer is disposed on the tunneling barrier layer. The stabilizing layer is disposed on the free layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims the prioritybenefit of an application Ser. No. 12/398,181, filed on Mar. 5, 2009,which claims the priority benefit of Taiwan application serial no.97142204, filed on Oct. 31, 2008. The entirety of each of theabove-mentioned patent applications is hereby incorporated by referenceherein and made a part of this specification.

BACKGROUND

1. Technical Field

The disclosure is related to a magnetic memory element, and inparticular to a magnetic memory element utilizing spin transferswitching.

2. Description of Related Art

The magnetic random access memory (MRAM) has the advantages such asnon-volatility, high density, high reading and writing speeds, andradiation resistance. When the conventional magnetic memory writes data,generally a magnetic memory cell selected by the intersection ofinduction magnetic fields of a write bit line (BL) and a write word line(WWL) is used to change a magnetoresistance (MR) of the magnetic memoryby changing a magnetized vector direction of the free layer. Whenreading a memory data, a reading current flows into the selectedmagnetic memory cell. Afterwards, a detected resistance of the magneticmemory cell is used to determine a digital value of the memory data.

A conventional magnetic memory cell is a stacked structure formed bystacking a pinning layer of an anti-ferromagnetic material, a pinnedlayer of ferromagnetic/non-magnetic metal/ferromagnetic layers, atunneling barrier layer and a free layer of a magnetic material. Throughthe high or low magnetoresistance (MR) derived from magnetizeddirections of the pinned layer and the free layer being in parallel oranti-parallel, data of a logic state “0” or a logic state “1” arerecorded.

As the CMOS technique keeps influencing advanced technology and in orderto respond to high-density MRAM designs, a magnetic tunnel junction(MTJ) element of the MRAM also continues shrinking in its size. For theconventional asteroid mode and the toggle mode MTJ elements, when theirsizes reduce, a switching field of the free layer in the MTJ elementscontinues to rise, and the operating margin reduces or even disappears.Therefore, how to enhance write selectivity and reduce write currentshave always been the most notable obstacles developing magnetic memorieshas ever encountered.

The spin torque transfer random access memory (STT-RAM) is considered asthe magnetic memory most likely to be applied in technology nodes beyond65 nm. For the STT-RAM utilizing spin transfer switching as its writemode, since write currents only pass through those selected memoryelements and magnetized switching depends upon write current density,shrinkage of elements is advantageous for reduction of write currents.In theory, enhancing write selectivity and reducing write currents canbe achieved simultaneously.

A theoretical estimated value of a switching critical current density ofthe magnetic memory element utilizing spin transfer switching isJc=αeM_(s)t[H_(k)+27πM_(s)]/hrη. M_(s) is a saturation magnetization ofa magnetic layer per unit volume; t is a thickness of the magneticlayer; H_(k) is an anisotropic field; a is the Gilbert damping constant;η is a spin polarization factor, and h is the Boltzmann constant. Theestimated value has not considered the effects of external electricfields and thermal dithering. It is known from the above formula that inresearch on the spin transmission effect, the factors influencing theswitching current mainly are the foregoing physical quantities(M_(s)H_(k), α and η. Among them, the saturation magnetization (M_(s))influences most significantly.

In U.S. Publication No. 2007/0159734 A1, an STT-RAM structure isproposed, which utilizes non-magnetic materials to perform doping in thefree layer, e.g., Cr, Cu, Au, B, Nb, Mo, Ta, Pt, Pd, Rh, Ru, Ag, TaN,CuN and TaCuN, thereby reducing the saturation magnetization (M_(s)) ofthe free layer so as to lower the critical current density (Jc).

However, in order to continue enhancing the performance of the STT-RAM,how to reduce the critical current density and maintain sufficientthermal stability in the STT-RAM are still urgent issues to be resolved.

SUMMARY

The disclosure provides another magnetic memory element utilizing spintransfer switching. The magnetic memory element includes a pinned layer,a tunneling barrier layer, a free layer and a stabilizing layer. Thetunneling barrier layer is disposed on the pinned layer. The free layeris disposed on the tunneling barrier layer. The stabilizing layer isdisposed on the free layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the firstembodiment of the disclosure.

FIG. 2 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the secondembodiment of the disclosure.

FIG. 3 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the thirdembodiment of the disclosure.

FIG. 4 is a curve diagram showing experiment results from a magneticmemory element utilizing spin transfer switching according to anexperiment of the disclosure.

FIG. 5 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the fourthembodiment of the disclosure.

FIG. 6 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the fifthembodiment of the disclosure.

FIG. 7 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the sixthembodiment of the disclosure.

FIG. 8 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the seventhembodiment of the disclosure.

DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the firstembodiment.

Referring to FIG. 1, a magnetic memory element 100 utilizing spintransfer switching includes a pinned layer 102, a tunneling barrierlayer 104 and a free layer structure. According to the presentembodiment, the free layer structure is, for example, a composite freelayer 106.

The pinned layer 102 is a stacked layer of ferromagnetic/non-magneticmetal/ferromagnetic material constituted by a lower pinned layer 108, acoupling layer 110 and an upper pinned layer 112. Magnetization vectorsof the lower pinned layer 108 and the upper pinned layer 112 arearranged as anti-parallel-coupled and not changed by operating magneticfields. It should be noted that the pinned layer 102 depicted in FIG. 1in the form of a multi-layered structure is provided for illustrationpurposes, and is not construed as limiting the scope of the disclosure.It is appreciated by persons skilled in the art that the pinned layercan be in the form of a single-layer structure.

The tunneling barrier layer 104 is disposed on the pinned layer 102. Amaterial of the tunneling barrier layer 104 is magnesium oxide (MgO) oraluminum oxide (AlO), for example.

The composite free layer 106 includes a first free layer 114, an insertlayer 116 and a second free layer 118. The first free layer 114 isdisposed on the tunneling barrier layer 104 and has a first spinpolarization factor and a first saturation magnetization. A material ofthe first free layer 114 is, for example, a magnetic material includinga magnetic alloy of Fe, Ni, Co, and B, such as CoFeB, CoFe, Co or Fe.The insert layer 116 is disposed on the first free layer 114. A materialof the insert layer 116 is a non-magnetic material, for example. Thematerial may be a metal material or a non-metal material, e.g., Ta, Ru,Mg, Ti, Cu, Cr, Ag, Pt, Pd, Au, tantalum oxide (TaO), rudium oxide(RuO), MgO or titanium oxide (TiO). The second free layer 118 isdisposed on the insert layer 116 and has a second spin polarizationfactor and a second saturation magnetization. A material of the secondfree layer 118 is, for example, a magnetic material, e.g., NiFe, Ni orCoFeB.

The first spin polarization factor is larger than the second spinpolarization factor, the second saturation magnetization is smaller thanthe first saturation magnetization, and a magnetization vector 120 ofthe first free layer 114 and a magnetization vector 122 of the secondfree layer 118 are arranged as parallel-coupled.

In addition, the magnetization vector 120 of the first free layer 114and the magnetization vector 122 of the second free layer 118 arearranged as parallel-coupled by an interlayer coupling through theinsert layer 116. The interlayer coupling includes a long rangecoupling, e.g., the Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, and ashort range coupling, e.g., a magnetostatic coupling.

Furthermore, a thickness of the insert layer 116 is, for example, athickness which renders the magnetization vector 120 of the first freelayer 114 and the magnetization vector 122 of the second free layer 118to be parallel-coupled. For example, when the insert layer 116 has acertain thickness, the RKKY coupling effect can be used to arrange themagnetization vector 120 of the first free layer 114 and themagnetization vector 122 of the second free layer 118 asparallel-coupled to each other. Moreover, when the thickness of theinsert layer 116 is of 1-6 angstroms or 12-18 angstroms, themagnetostatic coupling force can be used to arrange the magnetizationvector 120 of the first free layer 114 and the magnetization vector 122of the second free layer 118 as parallel-coupled.

Further, the magnetic memory element 100 utilizing spin transferswitching may optionally include a pinning layer 124. The pinned layer102 can be disposed on the pinning layer 124. A material of the pinninglayer 124 is an anti-ferromagnetic material, for example.

On the other hand, the magnetic memory element 100 utilizing spintransfer switching may further include a capping layer 126. The cappinglayer 126 is disposed on the composite free layer 106 and can be usedfor protecting the composite free layer 106. A material of the cappinglayer 126 is Ta, Ti, Ru, AlO, or MgO, for example.

Certainly, the magnetic memory element 100 utilizing spin transferswitching may further include a conductive line (not shown) electricallyconnected to the capping layer 126 and a conductive line (not shown)electrically connected to the pinning layer 124 so as to operate themagnetic memory element 100 utilizing spin transfer switching. A methodof operating the magnetic memory element 100 utilizing spin transferswitching is well-known to people having ordinary skill in the art andtherefore is not repeated herein.

It is known from the above embodiment that in the composite free layer106 of the magnetic memory element 100 utilizing spin transferswitching, since the first spin polarization factor of the first freelayer 114 is larger than the second spin polarization factor of thesecond free layer 118, the first free layer 114 can provide a high spinpolarization factor to obtain a high magnetoresistance ratio (MR %) sothat the reading signals are enhanced and the critical current density(Jc) is lowered effectively. Additionally, the second saturationmagnetization of the second free layer 118 is smaller than the firstsaturation magnetization of the first free layer 114, and therefore thesecond free layer 118 can assist the magnetization reversal of the firstfree layer 114 through the interlayer coupling and thus minimize thecritical current density. And a higher first saturation magnetization ofthe first free layer 114 is provided to maintain the sufficient thermalstability of the magnetic memory element.

It should be noted that the composite free layer 106 in the foregoingembodiment may also be applied to a magnetic memory element utilizingspin transfer switching whose free layer structure is a syntheticanti-ferromagnetic free layer. A magnetic memory element utilizing spintransfer switching whose free layer structure is a syntheticanti-ferromagnetic free layer is exemplified by the second embodimentand the third embodiment for illustration in the following.

FIG. 2 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the secondembodiment. FIG. 3 is a schematic cross-sectional view of the magneticmemory element utilizing spin transfer switching according to the thirdembodiment. The elements in FIGS. 2 and 3 which are the same as thoseappear in FIG. 1 use the same reference numerals, and a description ofsaid elements is not repeated herein.

Referring to both FIGS. 1 and 2, the difference between the secondembodiment and the first embodiment lies in that a free layer structure128 of a magnetic memory element 100′ utilizing spin transfer switchingof the second embodiment further includes a first spacer layer 130 and athird free layer 132 besides the composite free layer 106. The firstspacer layer 130 is disposed on the composite free layer 106, and amaterial of the first spacer layer 130 is Ru, Cr or Cu, for example. Thethird free layer 132 is disposed on the first spacer layer 130. Amaterial of the third free layer 132 is a magnetic material, forexample. A magnetization vector of the composite free layer 106 and amagnetization vector of the third free layer 132 are arranged asanti-parallel-coupled. In addition, materials and efficacies of theother components in the magnetic memory element 100′ utilizing spintransfer switching of the second embodiment are generally the same asthose in the magnetic memory element 100 utilizing spin transferswitching of the first embodiment, and therefore are not repeatedherein.

Referring to both FIGS. 2 and 3, the difference between the secondembodiment and the third embodiment lies in that in a magnetic memoryelement 100″ utilizing spin transfer switching of the third embodiment,the third free layer in a free layer structure 134 is also the compositefree layer 106. In other words, the free layer structure 134 in themagnetic memory element 100″ utilizing spin transfer switching includesfrom bottom upwards the tunneling barrier layer 104, the composite freelayer 106, the first spacer layer 130 and the composite free layer 106.Herein, magnetization vectors of the two composite free layers 106disposed over and under the first spacer layer 130 are arranged asanti-parallel to each other. Moreover, materials and efficacies of theother components in the magnetic memory element 100″ utilizing spintransfer switching of the third embodiment are generally the same asthose in the magnetic memory element 100′ utilizing spin transferswitching of the second embodiment, and therefore are not repeatedherein.

FIG. 4 is a curve diagram showing experiment results from the magneticmemory element utilizing spin transfer switching according to anexperiment of the disclosure.

In the magnetic memory element utilizing spin transfer switching of theexperiment, a material of the first free layer is CoFeB; a material ofthe second free layer is NiFe; a material of the insert layer is Ta andhas a very thin thickness (3 angstroms), and a material of the tunnelingbarrier layer is MgO. FIG. 4 shows a switching current density (Jc)under different write current pulses. A critial current density (J_(C0))writing from an anti-parallel high resistance (AP) to a parallel lowresistance (P) obtained by extrapolation is about 1.8×10⁶ A/cm², and acritical current density (J_(C0)) writing from a parallel low resistance(P) to an anti-parallel high resistance (AP) is about 2.3×10⁶ A/cm².Besides, a thermal stability (EB) from an anti-parallel high resistance(AP) to a parallel low resistance (P) obtained via a slope is about 57K_(b)T, and a thermal stability (EB) from a parallel low resistance (P)to an anti-parallel high resistance (AP) is about 53 K_(b)T.

FIG. 5 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the fourthembodiment. FIG. 6 is a schematic cross-sectional view of the magneticmemory element utilizing spin transfer switching according to the fifthembodiment. The elements shown in FIGS. 5 and 6 which are identical tothose appear in FIG. 1 are designated with the same reference numerals,and a description of said elements is not repeated hereinafter.

Referring to FIGS. 1 and 5 concurrently, the difference between thefourth embodiment and the first embodiment lies in that a magneticmemory element 400 utilizing spin transfer switching of the fourthembodiment further includes a stabilizing layer 136 and a second spacerlayer 138. The stabilizing layer 136 is disposed on the free layerstructure, and is located between the composite free layer 106 and thecapping layer 126. A material of the stabilizing layer 136 is, forexample, an anti-ferromagnetic material which may be PtMn, IrMn, or FeMnwith a thickness of greater 50 angstroms. The second spacer layer 138 isdisposed between the free layer structure and the stabilizing layer 136.A material of the second spacer layer 138 may be a metal material with aspin diffusion length greater than 100 angstroms, such as Ru, Ta, Ti,Cu, Au, Ag, Al, Pt or Pd. In addition, materials and efficacies of theother components in the magnetic memory element 400 utilizing spintransfer switching of the fourth embodiment are generally the same asthose in the magnetic memory element 100 utilizing spin transferswitching of the first embodiment, and therefore are not repeatedherein.

The stabilizing layer 136 provides an anisotropy to the free layerstructure without a significant exchange bias by means of adjusting thematerial and the thickness.

Accordingly, the anisotropy provided by the stabilizing layer 136 canincrease the coercivity of the free layer structure 106 to enhance thethermal stability. Furthermore, the stabilizing layer 136 may provide anadditional out-of-plane anisotropy and thereby reduce the requiredswitching current. The interfacial uncompensated moments of thestabilizing layer 136 which are coupled to and preferring aligned inparallel to the magnetization of the free layer structure 106, can alsoreduce the required switching current by reflecting the spin current toenhance the effectively spin transfer torque accordingly. In view of theabove, the magnetic memory element 400 utilizing spin transfer switchingcan effectively reduce the critical current density and improve thermalstability.

Referring to FIGS. 5 and 6 concurrently, the difference between thefifth embodiment and the fourth embodiment lies in that the secondspacer layer 138 presented in the magnetic memory element 400 utilizingspin transfer switching of the fourth embodiment is absent in a magneticmemory element 500 utilizing spin transfer switching of the fifthembodiment. Thus, in the magnetic memory element 500 utilizing spintransfer switching of the fifth embodiment, a stabilizing layer 536 isdisposed on the composite free layer 106, and directly contacts thesecond free layer 118. A thickness of the stabilizing layer 536 can bewithin a range of 10 angstroms to 50 angstroms. Besides, materials andefficacies of the other components in the magnetic memory element 500utilizing spin transfer switching of the fifth embodiment are generallythe same as those in the magnetic memory element 400 utilizing spintransfer switching of the fourth embodiment, and therefore are notrepeated herein.

The stabilizing layer 536 provides an anisotropy to the free layerstructure 106 without a significant exchange bias by means of adjustingthe material and the thickness. Accordingly, the anisotropy provided bythe stabilizing layer 536 can increase the coercivity of the free layerstructure 106 to enhance the thermal stability. Furthermore, thestabilizing layer 536 may provide an additional out-of-plane anisotropyand thereby reduce the required switching current. The interfacialuncompensated moments of the stabilizing layer 536 which are coupled toand preferring aligned in parallel to the magnetization of the freelayer structure 106, can also reduce the required switching current byreflecting the spin current to enhance the effectively spin transfertorque accordingly. Hence, the magnetic memory element 500 utilizingspin transfer switching can effectively reduce the critical currentdensity and improve thermal stability.

FIG. 7 is a schematic cross-sectional view of the magnetic memoryelement utilizing spin transfer switching according to the sixthembodiment. FIG. 8 is a schematic cross-sectional view of the magneticmemory element utilizing spin transfer switching according to theseventh embodiment. The elements shown in FIGS. 7 and 8 which areidentical to those appear in FIGS. 5 and 6 are designated with the samereference numerals, and a description of said elements is not repeatedhereinafter.

Referring to FIGS. 5 and 7 concurrently, the difference between thesixth embodiment and the fourth embodiment lies in that a free layer 606in a magnetic memory element 600 utilizing spin transfer switching ofthe sixth embodiment is in the of a single-layered structure and thecomposite free layer 106 in the magnetic memory element 400 utilizingspin transfer switching of the fourth embodiment is in the form of amulti-layered composite structure. A material of the free layer 606 maybe a magnetic material including a magnetic alloy of Fe, Ni, Co, and B,such as CoFeB, CoFe, NiFe, Ni, Co or Fe. In addition, materials andefficacies of the other components in the magnetic memory element 600utilizing spin transfer switching of the sixth embodiment are generallythe same as those in the magnetic memory element 400 utilizing spintransfer switching of the fourth embodiment, and therefore are notrepeated herein.

Likewise, the stabilizing layer 136 provides an anisotropy to the freelayer 606 without a significant exchange bias by means of adjusting thematerial and the thickness.

Accordingly, the anisotropy provided by the stabilizing layer 136 canincrease the coercivity of the free layer 606 to enhance the thermalstability. Furthermore, the stabilizing layer 136 may provide anadditional out-of-plane anisotropy and thereby reduce the requiredswitching current. The interfacial uncompensated moments of thestabilizing layer 136 which are coupled to and preferring aligned inparallel to the magnetization of the free layer 606, can also reduce therequired switching current by reflecting the spin current to enhance theeffectively spin transfer torque accordingly. Based on the foregoing,the magnetic memory element 600 utilizing spin transfer switching caneffectively reduce the critical current density and improve thermalstability.

Referring to FIGS. 6 and 8 concurrently, the difference between theseventh embodiment and the fifth embodiment lies in that a free layer706 in a magnetic memory element 700 utilizing spin transfer switchingof the seventh embodiment is in the of a single-layered structure andthe composite free layer 106 in the magnetic memory element 500utilizing spin transfer switching of the fifth embodiment is in the formof a multi-layered composite structure. A material of the free layer 706may be a magnetic material including a magnetic alloy of Fe, Ni, Co, andB, such as CoFeB, CoFe, NiFe, Ni, Co or Fe. Besides, materials andefficacies of the other components in the magnetic memory element 700utilizing spin transfer switching of the seventh embodiment aregenerally the same as those in the magnetic memory element 500 utilizingspin transfer switching of the fifth embodiment, and therefore are notrepeated herein.

Similarly, the stabilizing layer 536 provides an anisotropy to the freelayer 706 without a significant exchange bias by means of adjusting thematerial and the thickness. Accordingly, the anisotropy provided by thestabilizing layer 536 can increase the coercivity of the free layer 706to enhance the thermal stability. Furthermore, the stabilizing layer 536may provide an additional out-of-plane anisotropy and thereby reduce therequired switching current. The interfacial uncompensated moments of thestabilizing layer 536 which are coupled to and preferring aligned inparallel to the magnetization of the free layer 706, can also reduce therequired switching current by reflecting the spin current to enhance theeffectively spin transfer torque accordingly. Consequently, thereduction of the critical current density and the improvement of thermalstability can be achieved effectively in the magnetic memory element 700utilizing spin transfer switching.

In summary, the above embodiments have at least one of the followingadvantages:

The critical current density is effectively reduced by the magneticmemory elements utilizing spin transfer switching described in theforegoing embodiments.

The magnetic memory elements utilizing spin transfer switching of theforegoing embodiments have sufficient thermal stability.

The magnetic memory elements utilizing spin transfer switching of theforegoing embodiments have sufficient reading signals.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of thedisclosure without departing from the scope or spirit of the disclosure.In view of the foregoing, it is intended that the disclosure covermodifications and variations of this disclosure provided they fallwithin the scope of the following claims and their equivalents.

1. A magnetic memory element utilizing spin transfer switching, comprising: a pinned layer; a tunneling barrier layer, disposed on the pinned layer; a free layer, disposed on the tunneling barrier layer; and a stabilizing layer, disposed on the free layer.
 2. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, wherein a material of the tunneling barrier layer comprises magnesium oxide (MgO) or aluminum oxide.
 3. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, wherein a material of the free layer comprises a magnetic material.
 4. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, wherein a material of the stabilizing layer comprises an anti-ferromagnetic material.
 5. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, further comprising a pinning layer, wherein the pinned layer is disposed on the pinning layer.
 6. The magnetic memory element utilizing spin transfer switching as claimed in claim 5, wherein a material of the pinning layer comprises an anti-ferromagnetic material.
 7. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, further comprising a capping layer disposed on the stabilizing layer.
 8. The magnetic memory element utilizing spin transfer switching as claimed in claim 7, wherein a material of the capping layer is Ta, Ti, Ru, AlO_(x) or MgO.
 9. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, wherein the stabilizing layer has a thickness within a range of 10 angstroms to 50 angstroms.
 10. The magnetic memory element utilizing spin transfer switching as claimed in claim 1, further comprising a spacer layer disposed between the free layer and the stabilizing layer, wherein a material of the spacer layer comprises a metal with a spin diffusion length greater than 100 angstroms.
 11. The magnetic memory element utilizing spin transfer switching as claimed in claim 10, wherein the stabilizing layer has a thickness greater than 50 angstroms.
 12. The magnetic memory element utilizing spin transfer switching as claimed in claim 10, wherein the metal is Ru, Ta, Ti, Cu, Au, Ag, Al, Pt or Pd. 